Synthesis and Anticonvulsant Evaluation of some New 6-(Substituted-phenyl)thiazolo[3,2-b][1,2,4]triazole Derivatives in Mice

Epilepsy is the most frequent nearological affiction and afflicts 1% about of the world’s population. Currently there is an urgent need for the development of novel anticonvulsants with higher levels of potency and lower levels of toxicity. In this paper, a series of new 6-(substituted-phenyl)thiazolo[3,2-b][1,2,4]triazole derivatives were synthesized and tested for their anticonvulsant activities using the maximal electroshock (MES) and subcutaneous pentylenetetrazole (PTZ) screens, which are the most widely employed seizure models for early identification of candidate anticonvulsants. Their neurotoxicity was determined applying the rotarod test. In these compounds, 6-(4-fluorophenyl)thiazolo[3,2-b][1,2,4]triazole (3c) showed selective protection against the MES seizures with an ED50 value of 49.1 mg/Kg and a TD50 value of 94.1 mg/Kg, which provided compound 3c a protective index (PI = TD50/ED50) of 1.9 in the MES test. 6-(4-Propoxyphenyl)thiazolo[3,2-b][1,2,4]triazole (5b) was found to be active in both models, i.e. MES test and PTZ test. In the PTZ screen, compound 5b gave an ED50 of 63.4 mg/Kg and a TD50 of 105.6 mg/Kg, resulting in a PI value of 1.7 which is higher than carbamazepine.


Introduction
Epilepsy is one of the most common neurological disorders, which is characterized by excessive temporary neuronal discharge resulting in recurrent unprovoked seizures (1, 2). It has been reported that about 1% of the world's population (about 50 million people worldwide) are suffering with this neurological disorder at any one time (2). In recent years, significant efforts have been invested in the development of novel therapeutics, resulting in the emergence of several novel drugs as promising anticonvulsant agents (3, 4). However, the currently available anticonvulsants are effective in reducing the severity and frequency of seizures in less than 70% of patients. Up to 30% of patients are poorly treated with the available anticonvulsants (5,6). Moreover, their usage is often associated with numerous undesirable side-effects (7-12). High levels of toxicity and intolerance, and a lack of efficacy also represent further limitations of the current anticonvulsant agents. With all of this in mind, there is an urgent need for the development of novel antiepileptic drugs (AEDs) with higher levels of potency and lower levels of toxicity.
1,2,4-Triazoles represent a key structure motif ring to increase the hydrophobicity of the whole structure.
These compounds designed contained a hydrophobic unit (R), an electron donor group (D), and a hydrogen donor/acceptor unit (HAD), which are the major characteristics associated with good anticonvulsant activity for the currently used anticonvulsant agents (as shown in Figure 1) (27).

Materials and methods
Melting points were determined in open capillary tubes and were uncorrected. IR spectra were recorded (in KBr) on a IRPrestige-21. 1H-NMR spectra were measured on an AV-300 (Bruker, Fällanden, Switzerland), and all chemical shifts were given in ppm relative to tetramethysilane. Mass spectra were measured on an HP1100LC (Agilent Technologies, Santa Clara, CA, USA). Elemental analyses were performed on a 204Q CHN (Perkin Elmer, Fremont, CA, USA). The major chemicals were purchased from Aldrich Chemical Corporation (Shanghai, China).
6-(4-hydroxy-phenyl)thiazolo [3,2-b] mixture was added slowly 20 mL ice cold water and allowed to stir for half hour. After removing the dichloromethane under reduced pressure, the resulting white precipitate was obtained by filtration.

Pharmacology
Mail KunMing mice (supplied by the Laboratory of Animal Research, Yanbian University, China) weighting 18-22 g were used for pharmacological study. Animals were allowed free access to food and water except during the experiment and housed at controlled room temperature with 12 h light/dark schedule. All compounds were dissolved in dimethyl sulfoxide (DMSO) with the injection volume of 0.05 mL per 20 g, which had no effect on the test system.

Anticonvulsant effects in the MES test (29, 30)
The MES test was carried out using the methods described in the anticonvulsant drug development (ADD) program of the National Institutes of Health (USA). Seizures were elicited with a 60 Hz alternating current of 50 mA intensity in mice. The current was applied via corneal electrodes for 0.2 s. Protection against the spread of MES-induced seizures was defined as the abolition of the hind leg and tonic maximal extension component of the seizure. Animals were given intraperitoneal injection (i.p.) of the test compounds in the MES test. At 30 min after the administration of the compounds, the activities were evaluated in MES test. In phase-I screening, each compound was administered at the dose levels of 30, 100, and 300 mg/Kg for evaluating the preliminary anticonvulsant activity. For determination of the median effective dose (ED 50 ) and the median toxic dose (TD 50 ), the phase-II screening was prepared. Several groups (each group of 10 mice) were given various intraperitoneal doses of the tested compound until at least three points were established in the range of 10-90% seizure protection or neurotoxicity. The number of animals per group protected against MES (or neurotoxic in the rotarod test) is converted to a percentage, and a dose-response curve can be constructed. Then the respective ED 50 and TD 50 values, 95% confidence intervals were calculated by the statistics software SPSS 13.0 with probit analysis.

Neurotoxicity (NT) screening (29, 30)
The neurotoxicity of the compounds was measured in mice by the rotarod test. The mice were trained to stay on a rotarod of diameter 3.2 cm that rotates at 10 rpm. Trained animals were given i.p. of the test compounds. Neurotoxicity was indicated by the inability of the animal to maintain equilibrium on the rod for at least 1min in each of the trials.

PTZ-induced seizures (29, 30)
The PTZ test utilizes a dose of pentylenetetrazole (85 mg/kg). PTZ can produce clonic seizures lasting for at least five seconds in 97 percent of animals tested. At 30 min after the administration (i.p.) of the test compound, 85 mg/Kg PTZ dissolved in saline was administered subcutaneouly. Animals are observed over a 30 minute period. Absence of clonic spasms in the observed time period indicates that the compound has the ability to abolish the effect of pentylenetetrazole on seizure threshold.

Log P calculation
The calculated Log P (miLog P) values were calculated using the Molinspiration online property calculation toolkit (31).

Results and Discussion
Anticonvulsant activity A very important step in antiepileptic drug discovery is the choice of an appropriate animal model for the initial screening. At present, there are three models in-vivo -the MES, the PTZ, and the kindling model -which are routinely used by most AEDs discovery programs. Of these, the MES and PTZ seizure models represent the two animal seizure models most widely used in the search for new AEDs (32, 33). The MES test is thought to predict drugs effective against generalized seizures of the tonic-clonic (grand mal) type, whereas the PTZ test is used to find drugs effective against the generalized seizures of the petit mal (absence) type. In this study, the two models were used for screening the anticonvulsant activity of target compounds. In the preliminary evaluation of anticonvulsant activities (Phase I), doses of 30, 100, and 300 mg/Kg were used in both models, and the results were presented in Table 1.
All positive reaction numbers are in bold italic. a : Animals was administered intraperitoneal injection. b : Doses of 30, 100 and 300 mg/Kg were administered in maximal electroshock seizure test (MES), pentylenetetrazole-induced seizure test (PTZ), and neurotoxicity (NT) tests. c : The figures n/n indicate the number of animals protected/number of animals tested. The number of mice used is three. Sign "-" in the table means not tested.
According to the results of the anticonvulsant activity studies, 6-(4-fluorophenyl)thiazolo[3,2-b] [1,2,4]triazole (3c) was highly selective and found to be the most active compound in MES test with the complete protection at the dose of 100 mg/Kg and partial protection (one-third) at the dose of 30 mg/Kg. In the same test, compound 3a, 3b, 3k, 3l, and 5c were protective at the dose of 100 mg/Kg with the proportion of one-third, compound 5b showed protection in two-thirds at the same dose. Compounds 3a-3d, 3h-3l, and 5a-5d showed activities against MES at 300 mg/Kg in varying degrees.
It is well accepted that blood-brain barrier (BBB) is an important selective barrier on the drug›s way to the central nervous system. Overcoming the difficulty of delivering therapeutic agents to specific regions of the brain presents a major challenge to treatment of most brain disorders. According to Kaliszan et al. (34), lipophilicity (logP) and molecular weight (MW) of the compound are the main factors affecting drug delivery across the BBB. From the calculated LogP parameters of the prepared compounds (3a-3l, and 5a-5l), it can be observed that all compounds exhibited a nice LogP ranging from 2.1 to 5.1, which would enable the compounds to penetrate the BBB (Table 1). In group 5a-5l, however, only four compounds (5a-5d) with relatively small substituents showed anticonvulsant activities in MES or PTZ test, as the sizes of substituents increased, the anticonvulsant activity of them disappeared (5e-5l). This may be due to the big lipophilicity of the molecules, which interrupted the absorption and distribution of these compounds sequentially reduced bioavailability (35). The above considerations were also in agreement with the theory that there was an optimum Log P for the drugs acting on the central nervous system, and the drugs with this optimum Log P will be least inhibited in their movement through the aqueous and lipophilic phases of living tissue (36). Another contributor for the non-activity of compounds 5e-5l may be the steric hindrance formed by big size of the substitution, which may drop their affinity to some assumed target receptors.
From the rotarod test results, it seems that compounds, with anticonvulsant activities in MES/PTZ test, exhibited neurotoxicity at the same doses. For example, compounds 3c and 5b with high activity also displayed serious neurotoxicity.
Compounds 3c and 5b were selected for quantification of the pharmacological parameters (ED 50 and TD 50 ). Results of the quantitative test for the compounds, along with the data of the standard drugs carbamazepine, are reported in Table 2. In the MES screen, 6-(4-fluorophenyl)thiazolo[3,2-b][1,2,4] triazole (3c) showed an ED 50 and protective index (PI) value of 49.1 and 1.9. In the PTZ screen, 6-(4-propoxyphenyl)thiazolo[3,2-b] [1,2,4]triazole 5b gave an ED 50 of 63.4 mg/ Kg and a TD 50 of 105.6 mg/Kg, resulting in a high pi-value of 1.7 when compared to carbamazepine (PI < 0.44). : All animals was administered intraperitoneal injection. b : The median effective dose (ED 50 ) was measured in maximal electroshock seizure test, confidence intervals given in the bracket and the unit is mg/Kg. c : The median effective dose (ED 50 ) was measured in pentylenetetrazole-induced seizure test, confidence intervals given in the bracket and the unit is mg/Kg. d : The median neurotoxic dose (TD 50 ) was measured in the rotarod test, confidence intervals given in the bracket and the unit is mg/Kg. e : Not tested.

Conclusion
A series of new 6-(substitutedp h e n y l ) t h i a z o l o [ 3 , 2 -b ] [ 1 , 2 , 4 ] t r i a z o l e derivatives were synthesized and studied for their anticonvulsant activity using MES and PTZ tests. Among the compounds synthsized, two compounds (3c and 5b) were found to have promising anticonvulsant activities in the models employed for anticonvulsant evaluation. 6-(4-Fluorophenyl)thiazolo[3,2-b][1,2,4] triazole (3c) was highly selective and found to be the most active compound against MES seizures. 6-(4-Propoxyphenyl)thiazolo[3,2-b][1,2,4] triazole (5b) was active in both models. In the PTZ screen, compound 5b gave an ED 50 of 63.4 mg/Kg and a TD 50 of 105.6 mg/Kg, resulting in a high PI value of 1.7 when compared to carbamazepine (PI < 0.44). High Neurotoxicity is the main problem of this series of compounds, which resulted in the narrow safety margin. Further modifications of the thiazolo-triazole fragment will be the focus of our next efforts with the aim of reducing the neurotoxicity of these compounds.